ASCE/SEI 7 shake-up

NFPA Journal, Sep/Oct 2006 by Fleming, Russell P

Keeping pace with the earthquake protection provisions

NFPA CODES: NFPA 13, Installation of Sprinkler Systems

NFPA 13, Installation of Sprinkler Systems, first included provisions for the protection of sprinkler systems against earthquakes in 1947; at which point many building codes still didn't address the subject. In fact, the major changes in building code requirements for earthquake protection have come in the past twenty years with funding from the Federal Emergency Management Agency, leading to the development of the National Earthquake Hazards Reduction Program (NEHRP) provisions. The NEHRP provisions initially found their way into the codes through direct adoption, and now through reference to the American Society of Civil Engineer's standard ASCE/SEI 7, Minimum Design Loads for Buildings and Other Structures. The NFPA Committee on Hanging and Bracing of Water-Based Fire Protection Systems has been trying to keep pace with a moving target as successive editions of ASCE/SEI 7 address protection of architectural and mechanical systems.

In a Tentative Interim Amendment (TIA) to the 2002 edition of NFPA 13, effective August 6, 2003, and to an even greater extent in the amendments for the 2007 edition, changes were made to better coordinate the earthquake protection provisions of the sprinkler standard with those of ASCE/SEI 7.

The 2003 TIA to NFPA 13 reduced the allowable fastener loads to match current industry standards, limited the maximum spacing of lateral braces to 40 feet (12.2 meters) on center, and provided clarification of how NFPA 13 could be used in conjunction with the seismic design force formula contained in ASCE/SEI 7 for use with nonstructural components:

F^sub p^ = 0.4 a^sub p^ S^sub DS^ W^sub p^ (1 2(z/h)) / (R^sub p^/l^sub p^)

The design acceleration S^sub DS^ is based on widely available maps developed by the U.S. Geological Survey, and some of the other factors are directly assigned for fire sprinkler systems in ASCE/SEI 7. While the importance factor l^sub p^ has always been 1.5 for sprinkler systems, assigned values for some of the other factors changed between the 2002 and 2005 editions of ASCE/SEI 7. The TIA issued on the 2002 edition of NFPA 13 suggested the use of component response modification factor R^sub p^ of 3.5, correlating with the 2002 edition of ASCE/SEI 7, which also specified a component amplification factor a^sub p^ of 1.0. In the 2005 edition of ASCE/SEI 7 the R^sub p^ for a steel piping system is 4.5, and the a^sub p^ is 2.5.

The 2005 edition of ASCE/SEI 7 is referenced by the 2006 editions of the International Building Code® and NFPA 5000®, Building Construction and Safety Code® while the 2002 edition of ASCE/SEI 7 was referenced for use by the 2003 editions of those codes and the 2000 edition of the IBC. As such, compliance with the 2000 or 2003 editions of these codes would produce an a^sub p^ / R^sub p^ ratio of 1.0/3.5 = 0.29 whereas compliance with the 2006 editions of these codes produces a ratio of 2.5/4.5 = 0.55, resulting in significantly higher forces. To some extent, this has been offset by the clarification that the calculated design force can be reduced by a factor of 1.4 because ASCE/SEI 7 is based on strength design, whereas NFPA 13 uses allowable stress design. Prior to the 2007 edition, all loads in NFPA 13 were at allowable stress levels with the exception of the buckling loads for brace members. In the 2007 edition, tables that contained the allowable loads on braces have been reduced to add a factor of safety appropriate to the use of allowable stress design.

A simplified approach to determining seismic forces on sprinkler systems has also been developed for the 2007 edition of NFPA 13, involving a number of conservative assumptions. For example, the approach assumes poor soil conditions, leading to higher earthquake forces on the piping. The user of the standard always has the option to determine lateral forces in accordance with the actual equation of ASCE/SEI 7.

Because of the changes in ASCE/SEI 7, AHJs and others should be aware that the appropriate criteria for use in conjunction with the 2000 and 2003 editions of the model building codes are those found in the 2002 edition of NFPA 13, including the TIA. The appropriate criteria to be used with the 2006 editions of the model building codes are found in the 2007 edition of NFPA 13.

BY RUSSELL P. FLEMING, P.E.

RUSS FLEMING is the executive vice-president of the National Fire Sprinkler Association and a member of the NFPA Technical Correlating Committee on Automatic Sprinklers.